1. Field of the Invention
The present invention relates to an electronic governor system in which a control device performs an operation to open and close a throttle valve installed in an air suction system of an engine, thereby maintaining a desired engine rotation number, and the control device of the electronic governor system, and more particularly, it relates to an electronic governor system having a failure diagnosis function of detecting the occurrence of a failure in the system, and a control device of the electronic governor system.
2. Description of Related Art
In recent years, for the purpose of precisely controlling an engine, in place of an operator's accelerator operation to mechanically open and close the throttle valve, there has been spread an electronic governor system in which an electronic control device performs an operation to open and close a throttle valve, thereby controlling an engine rotation number. For example, an electronic governor system 1C having a constitution disclosed in JP-A-5-240073 and shown in FIG. 1 is well known, and has a hardware constitution which is common with an electronic governor system 1A of the present invention described later.
In the conventional electronic governor system 1C, an engine 2 which is a control object is subjected to various controls by an electronic control unit 10C, and a position (an open degree) of a throttle valve 32 incorporated in an electronic control throttle (an electronic governor) 30 disposed in an air suction passage 20 of the engine 2 is adjusted to change the amount of air to be sucked, thereby controlling an output of the engine.
In this case, as to the engine rotation number, an output signal from a crank angle sensor 12 disposed on the side of a crank shaft is taken into the electronic control unit 10C (F), and on the basis of the signal, the electronic control unit 10C operates a throttle actuator 13 for the throttle valve 32 disposed in the electronic control throttle 30, to control the engine rotation number. Moreover, the electronic control throttle 30 is provided with a throttle position sensor 14 so as to obtain positional information of the throttle valve 32, and an output signal from the sensor is also taken into the electronic control unit 10C, to execute control so that an actual engine rotation number converges to a desired constant rotation number on various load conditions.
FIG. 7 shows a simplified constitution of the electronic control throttle 30 seen from an air suction side. The butterfly type throttle valve 32 is disposed in a bore 31 which is an air passage, and a throttle shaft 33 connected to this valve rotates in a range of 90° around the shaft, to change a size of a space formed between the inner peripheral surface of the bore 31 and the throttle valve 32, thereby adjusting an air flow rate. Moreover, the throttle shaft 33 is connected to the throttle actuator 13 of a rotary driving system via gears 37, 38 and 39.
In the throttle actuator 13, a DC brush motor is usually used, and the electronic control unit 10C allows an arbitrary current to flow in a forward/backward direction on the basis of a pulse width modulation signal involving a polarity change (PWM excitation), thereby performing an operation to open and close the throttle valve 32. Moreover, when a target 34 attached to the end face of the throttle shaft 33 is detected by an encoder 35, the positional information of the throttle valve 32 is obtained, whereby servo control is executed so that an arbitrary throttle position is obtained on the basis of this positional information. It is to be noted that for the purpose of securing safety, a return spring 36 is disposed in the throttle shaft 33, and the throttle valve 32 is operated in a totally closed direction while the throttle actuator 13 is not operated owing to a failure or the like.
FIG. 8 shows an example of a control block diagram by software of the electronic control unit 10C which controls such hardware. P1 and P2 show plants (simulation models) of the throttle valve 32 and the engine 2. When the unit is actually operated, these plants become control objects, and these operation results are input into the electronic control unit 10C as an actual throttle position (θe) and an actual engine rotation number (Ne) in accordance with sensor outputs.
In a specific method of constantly executing the control in accordance with a desired arbitrary engine rotation number (Nd) on the basis of the above information, an error (Nerr) of the actual engine rotation number (Ne) with respect to the desired engine rotation number is calculated, and on the basis of this result, a block C2 of rotation number feedback control usually calculates a desired throttle position (θd). An error (θerr) of the actual throttle position (θe) with respect to this desired throttle position is calculated, and on the basis of this result, a block C1 of the feedback control obtains the throttle position, to calculate a control amount (U) of the throttle actuator 13. Then, when the throttle actuator 13 is driven on the basis of this control amount (U), the throttle position (θe) changes, and the amount of the air to be sucked changes, to increase or decrease a torque generated in the engine 2.
Moreover, various disturbance load torques (Tad) are added to the engine 2, but in accordance with a balance between these torques and the torque generated in the engine 2, the engine rotation number (Ne) is finally determined. Then, this engine rotation number (Ne) information is input into the block C2 for the rotation number control, and such a feedback control system executes the control so that the engine rotation number constantly converges to the desired engine rotation number (Nd). It is to be noted that in each feedback control calculation processing, an operation for allowing each error to converge to zero is performed, and in this operation, PID control, sliding mode control or the like is used.
However, in such conventional control, the failure of each sensor, the throttle actuator 13 or the like is not taken into consideration, and there has been a problem that if the failure occurs in each portion, the control becomes unstable, thereby causing engine stall, engine runaway or the like. To solve the problem, for example, as in a control device of an electronic governor system disclosed in JP-A-2003-214234, when a state where an absolute value of an error of a detected actual throttle open degree with respect to a desired throttle open degree is not less than a predetermined value continues for a predetermined time or longer, it is judged that the failure has occurred in a throttle control system.
However, even in a case where the failure of the throttle control system only is detected in this manner, if a failure occurs in another portion of the system, for example, as in the failure of the engine, this failure cannot be detected, whereby safety in the system is not sufficiently secured. Moreover, in the case of a serious failure involving the runaway, a failsafe control function to avoid at least the worst situation is preferably automatically exerted.